This invention relates to selectively plugged sinterable honeycomb structures and in particular, to a method and compositions for fabricating such structures.
A need for the present invention has arisen in the fabrication of solid particulate filter bodies from sinterable ceramic substrates. Filter bodies of the type being referred to are described and claimed in a pending application Ser. No. 165,646, entitled FILTER AND RELATED APPARATUS, filed July 3, 1980, which is assigned to the assignee of this application and is hereby incorporated by reference. Basically, the filter body is formed by providing a honeycomb structure having hollow open ended passages or cells extending between a pair of its opposing outer end faces. The cells are formed by thin, porous intersecting walls which also extend across and between the end faces. A subset of alternate cells are manifolded (i.e. plugged) or otherwise sealed in a checkered pattern at one of the end faces while the remaining alternate cells are sealed in a reversed checkered pattern at the remaining end face of the structure. A contaminated fluid, such as diesel engine exhaust gas, introduced under pressure to either of the end faces is forced into the cells open at that end face and through the thin, porous walls between those cells and the adjoining cells which open only at the remaining end face. The exhaust gas exits the filter body through these latter cells. Solid contaminants too large to pass through the open porosity of the thin walls are left behind in the filter body. The ceramic filter bodies are generally useful for moderately high temperature applications (up to at least 1000.degree. C.) and have been preferably formed from monolithically extruded and sintered honeycomb ceramic substrates having cordierite as their primary crystal phase as are described and claimed in U.S. Pat. Nos. 3,895,977 and 4,001,028 which exhibit particularly low coefficients of thermal expansion (15.times.10.sup.-6 /.degree.C. or less over the range 25.degree. to 1000.degree. C.). Foam-type ceramic cements, also having cordierite as their primary crystal phase, have been used to plug the cell ends of the substrates in fabricating the diesel particulate and other types of filter bodies and are described and claimed in another pending application Ser. No. 165,647 filed July 3, 1980 and entitled FOAM CERAMIC CEMENT originally issued as U.S. Pat. No. 4,297,140 and reissued as U.S. Pat. No. Re. 31,405, which is assigned to the assignee of this application and is incorporated by reference herein. These foam-type cements are used by first providing a sintered, cordierite honeycomb substrate, charging a formable batch of the cement into selected cell ends by conventional means, drying the cement plugs thus formed and then firing the substrate to sinter the cement. The foam-type cements contain an additive or additives, silicon carbide in the above-identified cordierite yielding cement, which react and generate gases at about the sintering temperature of the cement mixture. The released gases cause the remaining plug mixture components, which by this time have melted into a liquid or semi-liquid state, to foam and expand significantly in volume thereby filling all gaps between the plug and the surrounding thin walls forming the cells.
There are certain disadvantages in using the foam-type cordierite ceramic cements. For example, it would be desirable to form the filter body or other selectively plugged honeycomb structure in a single sintering step so as to minimize the time and energy required to fabricate the final structure. Use of the foam-type cements require two separate firing and cooling cycles: a first cycle to sinter the substrate and a second cycle after charging the selected cell ends to foam and sinter the cement. Also, it would be desirable to plug the substrates before their sintering as they typically experience some shrinkage during this step which increases the difficulty of subsequently charging a plugging material into their cell ends. Lastly, it would be desirable to plug the substrates with a material which could withstand the sintering temperature of the substrate as this would raise the maximum use temperature of the resultant plugged body. The range of firing temperatures of the foam-type cordierite cements are approximately several hundred degrees less than the sintering temperature of the preferred cordierite substrate material (e.g. approximately 1200.degree. C. versus 1400.degree. C., respectively). Use of the filter body above the lower firing temperature (approximately 1200.degree. C.) will have a deleterious effect on the foam-type cement plugs and could cause the loss of the cell end seal.
Although non-foaming ceramic compositions (i.e. cements) have been used in formable batch mixtures to bond together sections of green (i.e. dried but not sintered) ceramic honeycomb substrates, the cement and substrate sections all being sintered to primarily cordierite crystal phases during one firing operation, attempts to fabricate solid particulate filter bodies by charging a non-foaming ceramic material batch mixture into the cells of a green substrate extruded from the same or other ceramic batch mixtures were not successful as large numbers of cracks and voids would typically appear in the plugs and between the plugs and cell walls in the final sintered product. The foam-type cements have been heretofore preferred in fabricating these filter bodies as their foaming action has counteracted these problems.
Filter bodies might be fabricated by inserting solid, preformed green plugs into selected cell ends of a green honeycomb substrate and firing the plugs and substrate together in a manner similar to that described in U.S. Pat. No. 3,564,328, but that approach is undesirable for fabricating filter bodies. Because literally thousand of cells, must be plugged in fabricating the described filter bodies, it is easier to form the plugs using liquid or plastically formable plugging material which are introduced in bulk into many or all of the cell ends than to register and insert preformed plugs into the very small cell ends. Furthermore, the uncontrolled shrinkage difference taught by U.S. Pat. No. 3,564,328 results in imprecise outer dimensioning of the filter body and may cause fracturing of the thin porous walls forming the honeycomb matrix.